Real-time adaptive optics is a control process generally used to reduce optical phase error in an optical transmission system, such as a telescope or a laser beam director. In high bandwidth applications, such as astronomy and remote sensing, the goal of real-time adaptive optics is to operate at high frequencies to compensate for high bandwidth atmospheric distortion and to produce stable, high fidelity images. High-performance, real-time, adaptive optics systems include sensors, processors, and dynamic mirror assemblies, optionally including vibration compensation mirrors (i.e., fast steering mirrors that provide tip and tilt motions) and/or phase error compensation mirrors (i.e., deformable mirrors that provide surface figure adjustments). High-speed communications, high-bandwidth electronics, and high-bandwidth control systems are used to control both fast steering and deformable mirrors within the high-performance, real-time, adaptive optics systems.
The deformable mirror system is of primary concern in the present application and is discussed further hereafter. At the core of a typical deformable mirror is a thin mirror and a thick base plate, with electro-mechanical actuators positioned between and mechanically coupled to both the thin mirror and the thick base plate. Each actuator converts respective electrical signals into a respective mechanical force, which is used to deform the thin mirror locally. Depending on the deformable mirror application, the actuators may be designed to provide mechanical strokes that range from tens of nanometers to hundreds of microns, where actuation sensitivities are typically measured in nanometers or microns per volt, respectively.
Actuators used in deformable mirrors and micropositioner applications are typically ceramic, embodying piezoelectric, electrostrictive, or magnetostrictive properties of the ceramic material. A model of ceramic actuators includes both mechanical and electrical characteristics. Two well-known ceramic actuator formulations are PZT (lead-zirconate-titanate, classified as piezoelectric) and PMN (lead-magnesium-niobate, classified as electrostrictive). Design trade-off characteristics between actuator materials and their control systems include: thermal stability, creep, stiffness, dielectric constant, series and parallel resistance, and hysteresis.
Electrically, ceramic actuators may be modeled as capacitors. When multiple ceramic layers are stacked in parallel between alternating high- and low-side electrodes, the capacitance of the layers is added to determine the total actuator capacitance. In a co-fired stack process, tens of layers are typically stacked together, resulting in an actuator tens of millimeters in length and ranging into the tens of microfarads in capacitance. And, as in the case of a capacitor, actuators having higher capacitance require more energy (i.e., more current) to change the stored charge at a rate similar to an actuator of lesser capacitance. Thus, a high stroke requirement for an actuator results in a relatively high current to generate a high rate of change of the length of the actuator.
By way of example, in a high-performance, atmospheric compensation, adaptive optics system, a deformable mirror may employ actuators that are relatively long (e.g., 40 mm) to produce large mirror displacements (e.g., 4 μm). Because of the length, the actuators have comparable thickness (e.g., 10 mm) for strength. For PMN formulation actuators, these dimensions result in relatively high capacitance (e.g., 5 μf). Therefore, the amplifiers driving the actuators must provide sufficient drive power/current so as to achieve the bandwidths required by the high-performance adaptive optics system.
In most applications, high capacitance of an actuator tends to be a bad quality (e.g., high power requirements) with respect to operation of the system in which the actuator is deployed. On the other hand, the high capacitance of the actuator also tends to increase the time the actuator is capable of storing a charge, thereby maintaining its electrically or magnetically induced length change without requiring a constant source of power. Therefore, the high capacitance of the actuator may be a good quality in some applications, such as a long-exposure, low available-power, astronomical imaging application in a severe, but slowly changing, thermal environment.
In the past, the challenge for the adaptive optics community, specifically deformable mirror developers, has been to increase the bandwidths of traditional adaptive optics systems to correct optical phase error in high turbulence atmospheric conditions, which necessarily leads to driving a deformable mirror at higher frame rates and amplifiers and actuators at higher bandwidths.
For high-bandwidth applications, traditional deformable mirror driver electronics systems used to control the deformable mirror (DM) have one high-voltage amplifier per channel (i.e., each channel drives a single actuator) to achieve the high frame rates required to achieve the performance of the high-performance adaptive optics system. The amplifiers must have high output power to drive the actuators at the high frequencies and displacements required by the traditional adaptive optics system used in high turbulence conditions. Since the amplifiers that drive the actuators are traditionally linear, which have power efficiencies generally below 60%, then on a per actuator basis, the power required to drive a deformable mirror is high.
Deformable mirrors have been traditionally populated by 37, 97, 177, 349, 577, and 941 actuators. In future adaptive optics systems, the number of actuators populating a deformable mirror may be extended to up to 16,000 or more actuators per mirror. As discussed above, in traditional deformable mirror driver electronics systems, the number of actuators per mirror dictates the number of amplifiers per driver system. Thus, from the number of actuators in the deformable mirror, the driver system power, weight, size, and cost can be estimated.
Recently, adaptive optics systems have been considered for applications that do not require high bandwidths. Space telescopes, eye research, and nuclear fusion generation systems are examples in which low bandwidth adaptive optics systems are applicable. Low bandwidth adaptive optics systems do not require deformable mirrors to have high frame rates (i.e., the rate at which every actuator in the DM is addressed with a command update). Therefore, the associated DM driver electronics systems may also have reduced bandwidths and still support the frame rates necessary to achieve the portion of the error budget allotted to the DM within the adaptive optics system.
Low bandwidth applications raise issues related to the DM driver electronics that were not of serious concern in the high bandwidth applications, such as: size, weight, power consumption, packaging, radiation-hardening, and cost. These issues become increasingly important for deformable mirrors having actuator quantities in the thousands. Similar concerns are also raised when high bandwidth systems are to be used in weight- or power-limited environments, such as in airborne or space-based telescopes.